A classifier of this kind, as already known from German OS No. 1,607,631, is used for classifying, i.e. separating a mixture or quantity of granular material of different sizes and/or different shapes and/or different specific weight, into coarse material or coarse grains (of a grain-size above a certain diameter) and fine material (of a grain-size below that limit). The material to be classified is charged from above into a classifying area (an annular space between a vane-ring and a sifting rotor); at the same time, sifting air flows from the outside, through the vane-ring, into the classifying area, thus following a spiral path and carrying the particles along accordingly. In the classifying area, the particles are subjected to radially directed resistance and centrifugal forces acting in opposite directions. For a specific radial and axial air-velocity, there exists a limiting grain size at which the two forces are equal.
The coarser particles (coarser than the limiting size) thus arrive sooner or later at the coarse-material outlet at the bottom of the classifying area. The fine material, on the other hand, is carried along with the sifting air which flows inwardly through the passages in the sifting wheel and is drawn off through a fine-material/sifting-air outlet adjoining axially the interior of the sifting wheel. The fine material is finally separated from, or precipitated out of, the sifting air which can then be returned to the sifting-air inlet, i.e. it can be circulated. Classifiers of this kind are used mainly for recovering fine material of the smallest possible grain-size and in a narrow grain-size distribution, the upper limit sought being between 2 and 4 .mu.m. This very fine granular material is frequently used as a filler, for synthetic material, automobile tires, colours, paints or coating agents for paper. The coarse material is used for other purposes, or is re-ground to yield further fine material.
In order to obtain economical operation, it is therefore highly important that as much as possible of the fine material contained in the granular material charged into the classifier be separated as it passes therethrough, and that the throughput be relatively high and adjustable.